Described herein is a method and apparatus for heat treating and/or thermochemical, diffusional surface processing of metal articles or parts. More specifically, described herein is a method and an apparatus for nitriding metal articles, such as but not limited to, stainless and other, high-alloy steels as well as nickel or cobalt rich superalloys.
Austenitic stainless steels (SS) are highly valued for their corrosion-, oxidation-, and thermal-resistance, toughness and ductility, even at cryogenic temperatures. These steels contain high levels of chromium (Cr), as well as nickel (Ni) and/or manganese (Mn) that help stabilize their austenitic structure. The high levels of Cr and the other, easily oxidizing alloy additions, especially Al and Mn, that tend to form passive oxide films on metal surface can be also found in many grades of ferritic/martensitic, duplex, and precipitation hardening stainless steels, iron-, nickel- and cobalt-based superalloys, tool steels, bearing steels, and white cast irons. In order to enhance wear resistance, especially in the case of easily scratching austenitic SS and superalloys and, in some cases, increase both hardness and corrosion resistance, it is desired to treat and harden the surface using nitriding, an inexpensive, thermochemical-diffusional process well proven in the field of low-alloy and carbon steels. Unfortunately, the passive oxide films forming on metal surface act as dense diffusion barriers preventing the conventional nitriding. Table 1 compares the free energy of formation (Gibbs energy) of iron (Fe) oxides to the energy associated with the oxides of easily oxidizing alloying additions frequently found in stainless and tool steels as well as superalloys. All energies (per oxygen and/or metal atom) that are more negative than those associated with Fe-oxides indicate the propensity for the forming of passive oxide films inhibiting the conventional, and the most cost effective gas nitriding using ammonia (NH3) atmospheres.
TABLE 1Free Energy of Oxide Formation at 500° C.Delta G (kJ/O-g. at)Delta G (kJ/mol)mol) Energy perDelta G (kJ/M-g. at.)OxideEnergy per OxideOxygenEnergy per MetalFeO−214−214−214Fe3O4−860−215−287Fe2O3−616−205−308MnO−328−328−328Mn3O4−1,118−280−373Mn2O3−756−252−378Cr2O3−929−310−464V2O3−1,009−336−505V2O5−1,212−242−606V3O5−1,617−323−539NbO−349−349−349NbO2−653−326−653TiO−467−467−467TiO2−803−401−803ZrO2−952−476−952SiO2−770−385−770Al2O3−1,433−478−717Equilibrium Calculated using Software Package HSC Chemistry v. 5.0
Practical applications of metal alloys in corrosive and oxidizing environments, as well as practical observations of metal surface responses to various heat treating atmospheres or thermochemical treatments indicate that the highly alloyed, oxide film-passivating metal alloy articles contain at least 10.5 wt % Cr and at least 0.2 wt % of any of the following alloy additions in any combination or combined as a sum: Mn, Si, Al, V, Nb, Ti, and Zr.
Many methods have been developed to date in order to overcome the problem of passive oxide films during nitriding, nitrocarburizing and carbonitriding treatments in controlled atmosphere furnaces. Thus, the metal surface could be dry-etched at elevated temperatures in halide gases such as hydrochloric acid (HCl) or nitrogen trifluoride (NF3). This surface etching step, taking place in a corrosion resistant reactor equipped with toxic gas scrubbers, is immediately followed by nitriding or, alternatively, carburizing. Exposure to ambient air is avoided until the diffusion treatment is completed. The method is effective but requires a prolonged, multi-hour processing time, and necessitates significant capital, safety equipment, and maintenance expenditures. Process alternatives may include electrolytic etching and deposition of protective Ni-films preventing passive film formation. Of note, many legacy processes involved oxide dissolution and diffusional treatment in somewhat haphazard molten salts baths, typically containing very large quantities of liquid-phase, toxic cyanides.
Another, popular method involves low-pressure (vacuum furnace) nitriding using plasma ion glow discharges directly at the metal surface. Usually, this process takes more hours than gas nitriding in the ammonia atmospheres, its nitrogen deposition rate is comparably slow, and requires the metal parts to be one electrode with a conductive metal mesh suspended above the parts to be another. Ion sputtering action taking place in this process is sufficient to remove oxide films and enable the subsequent diffusional treatment. The key limitation is the part geometry—due to the configuration of mesh electrode, electrostatic fields formed and ion discharges directly over metal surface-treatment of parts containing holes, groves, or other special topographic features is difficult. Also, the cost of the entire system including high-power electric supplies, pumps and sealing is significant, temperature control of metal surface during the process is problematic due to ionic heating, and the thickness of nitrided case is comparatively low.
Thus, the metal processing industries need further improved thermochemical-diffusional treatments that will be capable of nitriding and surface hardening of stainless and other, high-alloy steels and superalloys in a cost-effective, safe, and rapid manner.